1. Field of the Invention
The present invention concerns a temperature-dependent switch having a bimetallic switching mechanism that is arranged in a housing having a first housing part and a second housing part, the bimetallic switching mechanism being connected to a first electrode that is guided out of one housing part and coacting with a second electrode that is provided internally on the other housing part.
2. Related Prior Art
A switch of this kind is known from WO 92/20086.
The known switch has a two-part housing made of insulating material, leading into each of whose opposing end faces is a stripped wire. A cavity, on whose bottom the one wire is arranged in a manner freely accessible from above, is provided in the interior of the housing.
A block on whose upper side the second wire ends is provided on the opposite end of the cavity, so that there is a difference in height between the two wire ends. A bimetallic switch tongue, which carries at its free end a movable contact which coacts with the end of the first wire, is attached at the end of the second wire. Depending on its temperature, the bimetallic switch tongue brings the movable contact into contact with the wire end or lifts it off therefrom.
The cover of the two-part housing is attached in hinged fashion to the lower part, and is intended to be welded by ultrasound to the lower part, thus achieving a tight seal with respect to the environment.
An advantage of the known switch is that it has small dimensions, and because of the insulating housing it has a low susceptibility to leakage currents but is not highly pressurestable. In addition, the air gaps in the interior of the housing are sufficiently large that the necessary insulating spacing is achieved.
Since the known switch is connected using leads, it can readily be processed further, but because the leads emerge from the housing at opposite ends, this switch is not well suited for automatic placement, since leads running parallel to one another are generally required in this context.
The known switch is associated with a whole series of further disadvantages, including principally the fact that current passes through the bimetallic switch tongue. The reason is that the electrical self-heating of the bimetallic switch tongue, which is in fact desirable here, influences the switching behavior in such a way that the switching temperature defined by the design of the bimetallic switch tongue can change as a function of current. In addition, the properties of the bimetallic switch tongue are influenced unpredictably by being welded onto the end of the second wire.
The known switch can therefore be used only to a limited extent for monitoring the temperature of a device being protected, since its switching temperature not only can change unpredictably as a result of the assembly operations, but also is influenced by the magnitude of the current that flows.
The known switch moreover has little contact reliability, because there are no geometrically reproducible contact conditions present between the bare wire end and the movable contact. This means that the contact surface depends in particular on the nature and magnitude of the curvature of the bimetallic switching tongue, which in turn is influenced in irreproducible fashion by mechanical stresses during welding and by the particular welding location itself. In addition, a special copper wire is necessary in order to provide resistance to abrasion.
A further disadvantage of the known switch consists in the fact that the housing is welded with ultrasound: the inventors of the present invention have recognized that in the case of the known switch, the use of ultrasound causes unpredictable changes in the switching behavior of the bimetallic switch tongue.
A further temperature-dependent switch that also has very small dimensions is known from DE-AS-2 121 802. This switch has a two-part metal housing made up of a lower part and a cover part insulated therefrom by means of an insulating film. The bimetallic switching mechanism consists in this case of a spring disk with a movable contact, and a bimetallic disk slipped over it.
The spring disk and the bimetallic disk are placed unconstrainedly into the lower part, i.e. are not subject to any mechanical loads. In the low-temperature position of the switching mechanism, the spring disk braces with its rim internally against the lower part, and presses the movable contact internally against the cover part, thus creating an electrically conductive connection between the lower part and the cover part. In this switch position, the bimetallic disk is completely unloaded.
When the temperature then rises above the changeover temperature of the bimetallic disk, it kicks over into its other configuration and then braces with its rim internally against the cover part, being electrically insulated with respect to the cover part by the interposed insulating film.
In its center region, the bimetallic disk now pushes the movable contact, against the force of the spring disk, away from the cover part and thereby opens the electrical connection between cover part and lower part.
With this switching mechanism, the predominating disadvantages present in the case of the switch known from WO 92/20086 do not exist. The bimetallic disk is not subject to any mechanical stresses when it is in its low-temperature position, and is furthermore not responsible for leading electrical current, so that no electrical self-heating occurs. On the other hand, the spring disk is responsible only for creating the electrical contact and taking over the current, so that the spring disk can be designed substantially with an eye to good current-leading properties. As a result, the electrical properties and the properties responsible for switching over as a function of temperature can, with this switch, be adjusted separately from one another; these properties moreover cannot be altered by assembly of the known switch.
This switch also has certain disadvantages, however, associated with the fact that it has a two-part metal housing. On the one hand, problems can occur during assembly of the known switch if the insulating film slips, so that the necessary insulation between lower part and cover part is not achieved, or the bimetallic disk is not sufficiently insulated, in its high-temperature position, with respect to the cover part. Particular problems occur here with leakage currents and with air gaps that are insufficient for insulation.
The housing of the known switch is furthermore not sufficiently sealed for certain applications: the cover part is held on the lower part merely by a crimped edge, and the insulating film provides sealing, which is nevertheless not always sufficient and may indeed be absent in the event of misassembly.
A further disadvantage of the admittedly very pressure-resistant metal housing consists in the fact that in many applications, it must still be insulated with respect to the device being protected. In the case of the known switch, the connection technology is such that crimp terminals are provided on both housing parts, to which terminals the connecting leads must then be attached by the user; because this cannot be automated, it is often also regarded as a disadvantage.
DE 43 37 141 A1 discloses a similarly configured switch which has the same advantages as the switch known from DE-AS-2 121 802. In this switch, however, the insulating film is adhesively bonded onto the cover part prior to assembly, so that the disadvantages associated with slippage of the insulating film are avoided.
This switch additionally has an external shoulder on the housing, on which sits one annular end of a connecting lug to whose other end a first connecting lead is soldered. The second connecting lead is soldered directly onto the cover part.
Although most of the aforementioned disadvantages have been eliminated with this switch, the technology for connecting the leads to the switch is nevertheless very complex, which is also regarded as a disadvantage. This switch moreover has relatively large dimensions.
It is a general disadvantage with both of the above-described switches having metal housings that the necessary protection from leakage currents and the necessary size of the air gaps is often not present for the connecting contacts, or has already been utilized to such an extent that the known switches cannot be further miniaturized.